[Forms] Begin adding controller for selector

Begin implementing controller to populate the domain
object selector, added for WTD-987.

platform/commonUI/general/src/controllers/SelectorController.js

@@ -0,0 +1,56 @@
+/*global define*/
+
+define(
+    [],
+    function () {
+        "use strict";
+
+        /**
+         * Controller for the domain object selector control.
+         * @constructor
+         * @param {ObjectService} objectService service from which to
+         *        read domain objects
+         * @param $scope Angular scope for this controller
+         */
+        function SelectorController(objectService, $scope) {
+            var treeModel = {},
+                previousSelected;
+
+            // For watch; look at the user's selection in the tree
+            function getTreeSelection() {
+                return treeModel.selectedObject;
+            }
+
+            // Get the value of the field being edited
+            function getField() {
+                return $scope.ngModel[$scope.field];
+            }
+
+            // Check that a selection is of the valid type
+            function validateTreeSelection(selectedObject) {
+                var type = selectedObject &&
+                    selectedObject.getCapability('type');
+
+                // Delegate type-checking to the capability...
+                if (!type || !type.instanceOf($scope.structure.type)) {
+                    treeModel.selectedObject = previousSelected;
+                }
+
+                // Track current selection to restore it if an invalid
+                // selection is made later.
+                previousSelected = treeModel.selectedObject;
+            }
+
+            // Reject attempts to select objects of the wrong type
+            $scope.$watch(getTreeSelection, validateTreeSelection);
+
+            return {
+                // Expose tree model for use in template directly
+                treeModel: treeModel
+            };
+        }
+
+
+        return SelectorController;
+    }
+);